Evidence from d+ Au measurements for final-state suppression of high ...

arXiv:nucl-ex/0306024v3 18 Aug 2003

Evidence from d+Au measurements for final-state suppression of high pT hadrons in Au+Au collisions at RHIC J. Adams,3 C. Adler,12 M.M. Aggarwal,25 Z. Ahammed,28 J. Amonett,17 B.D. Anderson,17 M. Anderson,5 D. Arkhipkin,11 G.S. Averichev,10 S.K. Badyal,16 J. Balewski,13 O. Barannikova,28, 10 L.S. Barnby,17 J. Baudot,15 S. Bekele,24 V.V. Belaga,10 R. Bellwied,41 J. Berger,12 B.I. Bezverkhny,43 S. Bhardwaj,29 P. Bhaskar,38 A.K. Bhati,25 H. Bichsel,40 A. Billmeier,41 L.C. Bland,2 C.O. Blyth,3 B.E. Bonner,30 M. Botje,23 A. Boucham,34 A. Brandin,21 A. Bravar,2 R.V. Cadman,1 X.Z. Cai,33 H. Caines,43 M. Calder´ on de la Barca S´ anchez,2 J. Carroll,18 18 41 5 9 38 J. Castillo, M. Castro, D. Cebra, P. Chaloupka, S. Chattopadhyay, H.F. Chen,32 Y. Chen,6 S.P. Chernenko,10 M. Cherney,8 A. Chikanian,43 B. Choi,36 W. Christie,2 J.P. Coffin,15 T.M. Cormier,41 J.G. Cramer,40 H.J. Crawford,4 D. Das,38 S. Das,38 A.A. Derevschikov,27 L. Didenko,2 T. Dietel,12 X. Dong,32, 18 J.E. Draper,5 F. Du,43 A.K. Dubey,14 V.B. Dunin,10 J.C. Dunlop,2 M.R. Dutta Majumdar,38 V. Eckardt,19 L.G. Efimov,10 V. Emelianov,21 J. Engelage,4 G. Eppley,30 B. Erazmus,34 P. Fachini,2 V. Faine,2 J. Faivre,15 R. Fatemi,13 K. Filimonov,18 P. Filip,9 E. Finch,43 Y. Fisyak,2 D. Flierl,12 K.J. Foley,2 J. Fu,42 C.A. Gagliardi,35 M.S. Ganti,38 N. Gagunashvili,10 J. Gans,43 L. Gaudichet,34 M. Germain,15 F. Geurts,30 V. Ghazikhanian,6 P. Ghosh,38 J.E. Gonzalez,6 O. Grachov,41 V. Grigoriev,21 S. Gronstal,8 D. Grosnick,37 M. Guedon,15 S.M. Guertin,6 A. Gupta,16 E. Gushin,21 T.D. Gutierrez,5 T.J. Hallman,2 D. Hardtke,18 J.W. Harris,43 M. Heinz,43 T.W. Henry,35 S. Heppelmann,26 T. Herston,28 B. Hippolyte,43 A. Hirsch,28 E. Hjort,18 G.W. Hoffmann,36 M. Horsley,43 H.Z. Huang,6 S.L. Huang,32 T.J. Humanic,24 G. Igo,6 A. Ishihara,36 P. Jacobs,18 W.W. Jacobs,13 M. Janik,39 I. Johnson,18 P.G. Jones,3 E.G. Judd,4 S. Kabana,43 M. Kaneta,18 M. Kaplan,7 D. Keane,17 J. Kiryluk,6 A. Kisiel,39 J. Klay,18 S.R. Klein,18 A. Klyachko,13 D.D. Koetke,37 T. Kollegger,12 A.S. Konstantinov,27 M. Kopytine,17 L. Kotchenda,21 A.D. Kovalenko,10 M. Kramer,22 P. Kravtsov,21 K. Krueger,1 C. Kuhn,15 A.I. Kulikov,10 A. Kumar,25 G.J. Kunde,43 C.L. Kunz,7 R.Kh. Kutuev,11 A.A. Kuznetsov,10 M.A.C. Lamont,3 J.M. Landgraf,2 S. Lange,12 C.P. Lansdell,36 B. Lasiuk,43 F. Laue,2 J. Lauret,2 A. Lebedev,2 R. Lednick´ y,10 V.M. Leontiev,27 M.J. LeVine,2 C. Li,32 Q. Li,41 S.J. Lindenbaum,22 M.A. Lisa,24 F. Liu,42 42 L. Liu, Z. Liu,42 Q.J. Liu,40 T. Ljubicic,2 W.J. Llope,30 H. Long,6 R.S. Longacre,2 M. Lopez-Noriega,24 W.A. Love,2 T. Ludlam,2 D. Lynn,2 J. Ma,6 Y.G. Ma,33 D. Magestro,24 S. Mahajan,16 L.K. Mangotra,16 D.P. Mahapatra,14 R. Majka,43 R. Manweiler,37 S. Margetis,17 C. Markert,43 L. Martin,34 J. Marx,18 H.S. Matis,18 Yu.A. Matulenko,27 T.S. McShane,8 F. Meissner,18 Yu. Melnick,27 A. Meschanin,27 M. Messer,2 M.L. Miller,43 Z. Milosevich,7 N.G. Minaev,27 C. Mironov,17 D. Mishra,14 J. Mitchell,30 B. Mohanty,38 L. Molnar,28 C.F. Moore,36 M.J. Mora-Corral,19 V. Morozov,18 M.M. de Moura,41 M.G. Munhoz,31 B.K. Nandi,38 S.K. Nayak,16 T.K. Nayak,38 J.M. Nelson,3 P. Nevski,2 V.A. Nikitin,11 L.V. Nogach,27 B. Norman,17 S.B. Nurushev,27 G. Odyniec,18 A. Ogawa,2 V. Okorokov,21 M. Oldenburg,18 D. Olson,18 G. Paic,24 S.U. Pandey,41 S.K. Pal,38 Y. Panebratsev,10 S.Y. Panitkin,2 A.I. Pavlinov,41 T. Pawlak,39 V. Perevoztchikov,2 W. Peryt,39 V.A. Petrov,11 S.C. Phatak,14 R. Picha,5 M. Planinic,44 J. Pluta,39 N. Porile,28 J. Porter,2 A.M. Poskanzer,18 M. Potekhin,2 E. Potrebenikova,10 B.V.K.S. Potukuchi,16 D. Prindle,40 C. Pruneau,41 J. Putschke,19 G. Rai,18 G. Rakness,13 R. Raniwala,29 S. Raniwala,29 O. Ravel,34 R.L. Ray,36 S.V. Razin,10, 13 D. Reichhold,28 J.G. Reid,40 G. Renault,34 F. Retiere,18 A. Ridiger,21 H.G. Ritter,18 J.B. Roberts,30 O.V. Rogachevski,10 J.L. Romero,5 A. Rose,41 C. Roy,34 L.J. Ruan,32, 2 V. Rykov,41 R. Sahoo,14 I. Sakrejda,18 S. Salur,43 J. Sandweiss,43 I. Savin,11 J. Schambach,36 R.P. Scharenberg,28 N. Schmitz,19 L.S. Schroeder,18 K. Schweda,18 J. Seger,8 D. Seliverstov,21 P. Seyboth,19 E. Shahaliev,10 M. Shao,32 M. Sharma,25 K.E. Shestermanov,27 S.S. Shimanskii,10 R.N. Singaraju,38 F. Simon,19 G. Skoro,10 N. Smirnov,43 R. Snellings,23 G. Sood,25 P. Sorensen,6 J. Sowinski,13 H.M. Spinka,1 B. Srivastava,28 S. Stanislaus,37 R. Stock,12 A. Stolpovsky,41 M. Strikhanov,21 B. Stringfellow,28 C. Struck,12 A.A.P. Suaide,41 9 ˇ E. Sugarbaker,24 C. Suire,2 M. Sumbera, B. Surrow,2 T.J.M. Symons,18 A. Szanto de Toledo,31 P. Szarwas,39 6 31 2, 23 A. Tai, J. Takahashi, A.H. Tang, D. Thein,6 J.H. Thomas,18 V. Tikhomirov,21 M. Tokarev,10 M.B. Tonjes,20 T.A. Trainor,40 S. Trentalange,6 R.E. Tribble,35 M.D. Trivedi,38 V. Trofimov,21 O. Tsai,6 T. Ullrich,2 D.G. Underwood,1 G. Van Buren,2 A.M. VanderMolen,20 A.N. Vasiliev,27 M. Vasiliev,35 S.E. Vigdor,13 Y.P. Viyogi,38 S.A. Voloshin,41 W. Waggoner,8 F. Wang,28 G. Wang,17 X.L. Wang,32 Z.M. Wang,32 H. Ward,36 J.W. Watson,17 R. Wells,24 G.D. Westfall,20 C. Whitten Jr.,6 H. Wieman,18 R. Willson,24 S.W. Wissink,13 R. Witt,43 J. Wood,6 J. Wu,32 N. Xu,18 Z. Xu,2 Z.Z. Xu,32 A.E. Yakutin,27 E. Yamamoto,18 J. Yang,6 P. Yepes,30 V.I. Yurevich,10 Y.V. Zanevski,10 I. Zborovsk´ y,9 H. Zhang,43, 2 H.Y. Zhang,17

2 ˙ lnierczuk,13 R. Zoulkarneev,11 J. Zoulkarneeva,11 and A.N. Zubarev10 W.M. Zhang,17 Z.P. Zhang,32 P.A. Zo (STAR Collaboration), ∗ 1

Argonne National Laboratory, Argonne, Illinois 60439 Brookhaven National Laboratory, Upton, New York 11973 3 University of Birmingham, Birmingham, United Kingdom 4 University of California, Berkeley, California 94720 5 University of California, Davis, California 95616 6 University of California, Los Angeles, California 90095 7 Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 8 Creighton University, Omaha, Nebraska 68178 9 ˇ z/Prague, Czech Republic Nuclear Physics Institute AS CR, Reˇ 10 Laboratory for High Energy (JINR), Dubna, Russia 11 Particle Physics Laboratory (JINR), Dubna, Russia 12 University of Frankfurt, Frankfurt, Germany 13 Indiana University, Bloomington, Indiana 47408 14 Insitute of Physics, Bhubaneswar 751005, India 15 Institut de Recherches Subatomiques, Strasbourg, France 16 University of Jammu, Jammu 180001, India 17 Kent State University, Kent, Ohio 44242 18 Lawrence Berkeley National Laboratory, Berkeley, California 94720 19 Max-Planck-Institut f¨ ur Physik, Munich, Germany 20 Michigan State University, East Lansing, Michigan 48824 21 Moscow Engineering Physics Institute, Moscow Russia 22 City College of New York, New York City, New York 10031 23 NIKHEF, Amsterdam, The Netherlands 24 Ohio State University, Columbus, Ohio 43210 25 Panjab University, Chandigarh 160014, India 26 Pennsylvania State University, University Park, Pennsylvania 16802 27 Institute of High Energy Physics, Protvino, Russia 28 Purdue University, West Lafayette, Indiana 47907 29 University of Rajasthan, Jaipur 302004, India 30 Rice University, Houston, Texas 77251 31 Universidade de Sao Paulo, Sao Paulo, Brazil 32 University of Science & Technology of China, Anhui 230027, China 33 Shanghai Institute of Nuclear Research, Shanghai 201800, P.R. China 34 SUBATECH, Nantes, France 35 Texas A & M, College Station, Texas 77843 36 University of Texas, Austin, Texas 78712 37 Valparaiso University, Valparaiso, Indiana 46383 38 Variable Energy Cyclotron Centre, Kolkata 700064, India 39 Warsaw University of Technology, Warsaw, Poland 40 University of Washington, Seattle, Washington 98195 41 Wayne State University, Detroit, Michigan 48201 42 Institute of Particle Physics, CCNU (HZNU), Wuhan, 430079 China 43 Yale University, New Haven, Connecticut 06520 44 University of Zagreb, Zagreb, HR-10002, Croatia (Dated: February 8, 2008;) 2

We report measurements of single-particle inclusive spectra and two-particle azimuthal distributions of charged hadrons at high transverse momentum (high pT ) in minimum bias and central √ d+Au collisions at sNN =200 GeV. The inclusive yield is enhanced in d+Au collisions relative to binary-scaled p+p collisions, while the two-particle azimuthal distributions are very similar to those observed in p+p collisions. These results demonstrate that the strong suppression of the inclusive yield and back-to-back correlations at high pT previously observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions. PACS numbers: 25.75.-q, 25.75.Dw,25.75.Gz

Energetic partons propagating through matter are predicted to lose energy through induced gluon radiation, with the magnitude of the energy loss depending strongly on the color charge density [1]. Partonic energy loss is

potentially a sensitive probe of the matter created in high energy heavy-ion collisions, where a quark-gluon plasma may form if sufficiently high energy density is achieved. The energetic partons originate in the hard scattering of

partons from the incoming nuclei. Direct measurement of jets resulting from parton fragmentation is difficult in nuclear collisions; nevertheless partonic energy loss can be studied using observables such as inclusive spectra and two-particle azimuthal distributions of high transverse momentum (high pT ) hadrons. Measurements of high pT hadron production in ultrarelativistic interactions of heavy nuclei reveal strong suppression of both the single-particle inclusive yield [2, 3, 4, 5] and back-to-back pairs (large azimuthal separation ∆φ) in the most-central, violent collisions, while near-side pairs (small ∆φ) exhibit jet-like correlations that are similar to those in proton+proton (p+p) collisions [6]. One interpretation of these results is that, in the final state following the hard scattering, energetic partons traversing the dense medium in the core of the collision lose energy, and the observed jets are primarily those created from partons produced near the surface and directed outwards [6]. Alternatively, the suppression might result from initial-state effects prior to the hard scattering, such as the saturation of gluon densities in the incoming nuclei [7]. Models incorporating either picture are capable of describing central Au+Au collision data [5]. Initial- and final-state effects in Au+Au collisions can be separated through studies of deuteron(d)+Au collisions. Theoretical expectations for d+Au collisions at the Relativistic Heavy Ion Collider (RHIC) are given in [7, 8, 9, 10, 11, 12, 13, 14, 15]. Within a perturbative QCD (pQCD) framework, the expected initial-state nuclear effects in d+Au collisions are multiple scattering prior to a hard collision, which has been used to explain the Cronin enhancement of the inclusive yield [16], and shadowing of the parton distribution functions. Nuclear effects are expected to increase for more central collisions; thus the centrality dependence of observables measured in d+Au collisions also will help reveal their origin. The STAR Collaboration reports measurements of the inclusive invariant pT distribution and two-particle azimuthal distributions at high pT for charged hadrons [(h+ + h− )/2, approximated by the summed yields of primary π ± , K± , p and p ¯ ] in minimum bias and central √ d+Au collisions at center of mass energy sNN =200 GeV per nucleon pair. Comparison is made to measurements √ at sNN =200 GeV in the same detector for Au+Au and p+p interactions [5, 6]. The inclusive yield is enhanced in d+Au collisions relative to binary-scaled p+p collisions, in contrast to the large suppression observed in central Au+Au interactions. Similar results are reported in [17, 18, 19]. The d+Au two-particle azimuthal distributions are very similar to those observed in p+p collisions. These observations are consistent with expectations from pQCD models incorporating both the Cronin enhancement and nuclear shadowing [8, 9, 10, 11, 12], and are inconsistent with calculations that attribute the suppression in central Au+Au collisions to initial-state gluon saturation [7].

1 dN/dN (FTPC-Au) ch Nevents

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FIG. 1: Uncorrected charged particle multiplicity distributions measured in −3.8 < η < −2.8 (Au-direction) for d+Au collisions. Points are for minimum bias (triangles) and peripheral (circles, ZDC-d single neutron) collisions. Both are normalized to the total number of d+Au collisions. Histograms are Glauber model calculations.

STAR is a multi-purpose detector [20] located at Brookhaven National Laboratory’s RHIC facility. For these measurements, the minimum bias trigger required at least one beam-rapidity neutron in ZDC-Au, the Zero Degree Calorimeter (ZDC) in the Au beam direction, which is assigned negative pseudorapidity (η). This trigger accepts 95±3% of the d+Au hadronic cross section dAu σhadr . Trigger backgrounds were measured using beam bunches not in collision. Charged particle momenta were measured by the Time Projection Chamber in a 0.5 T solenoidal magnetic field. After event selection cuts, the data set consists of 107 minimum bias d+Au events. Data were analyzed using the techniques described in [2, 6]. The vertex was reconstructed in 93±1% of triggered minimum bias events. The spectra were corrected for trigger and vertex-finding efficiencies. Contamination of the spectra due to weak decay products was corrected based on HIJING [21]. Results of an independent analysis, using a different technique for vertex reconstruction [5], agree with the reported spectrum within the relative normalization uncertainties at all pT . Centrality tagging of d+Au collisions is based on the raw (uncorrected) charged particle multiplicity within −3.8 < η < −2.8, measured by the Forward Time Projection Chamber in the Au beam direction (FTPC-Au [20]). The FTPC-Au multiplicity was examined in quadrants relative to the orientation of the leading charged hadron at mid-rapidity; auto-correlation effects were found to be negligible. An independent centrality tag, used as a cross-check, requires at least one beam-rapidity (spectator) neutron in ZDC-d, the ZDC in the deuteron beam direction. The cross section for this process in hadronic dAu . ZDC-d events was measured to be (19.2±1.3)% of σhadr

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FIG. 2: Inclusive pT distributions for minimum bias and central d+Au collisions, and non-singly diffractive p+p collisions [5]. Hash marks at the top indicate bin boundaries for pT >3.8 GeV/c.

RAB (pT)

and FTPC-Au are separated by 8 rapidity units. Figure 1 shows the FTPC-Au multiplicity for minimum bias and ZDC-d neutron-tagged events. The latter have a strong bias toward low multiplicity. The centrality tags were modeled using a Monte Carlo Glauber calculation [2] incorporating the Hulth´en wavefunction of the deuteron[22]. In this model the mean number of binary collisions hNbin i is 7.5±0.4 for minidAu mum bias events and σhadr =2.21±0.09 b. Events with a neutron spectator from the deuteron comprise (18±3)% dAu of σhadr in the model. This event class is biased toward peripheral collisions, with hNbin i=2.9±0.2. The FTPC-Au multiplicity distribution was modeled by convoluting the Glauber model distribution of participants from the Au nucleus with the charged multiplicity distribution measured in 2.5